This invention relates to filters of electromagnetic signals, such as microwave signals, wherein the filter provides a bandpass function characterized by a multiple-pole transmission band and, more particularly, to the construction of a filter having a miniaturized configuration wherein a plurality of resonators of planar form are stacked one above the other to provide for plural modes of electromagnetic vibration within a single resonator. The resonators are spaced apart and supported by dielectric material with irises disposed between the resonators for coupling electromagnetic power among the resonators.
Filters are employed in numerous circuits for signal processing, communication, and other functions. Of particular interest herein are circuits, such as those which may be constructed on a printed circuit board, and are operable at microwave frequencies, such as frequencies in the gigahertz region. Such signals may be processed by transistors and other solid state devices, and may employ analog filters in the form of a series of cavity resonators, or resonators configured in microstrip form. By way of example, to provide a band-pass filter having an elliptic function or a Chebyshev response, and wherein a mathematical representation of the response is characterized by numerous poles, the filter has many sections. Each section has a single resonator, in the microstrip form of circuit, for each pole which is to be produced in the filter transfer function.
A problem arises in that the circuitry may be unduly large because of the space occupied by one or more filters employed in the circuitry. In the case of cavity-resonator filters, each cavity is physically large, particularly at lower frequencies, the physical size militating against the use of the cavity filters. Thus, in situations wherein there is limited space available for electronic circuits, such as in satellites which serve as part of a communication system, there is a need to reduce the size of filters, as well as to decrease the weight of filters employed in the signal processing circuitry.